This invention relates to a tapping screw, and more specifically a tapping screw for use e.g. in securing an outer wall to a light gage steel plate of a steel structure.
A conventional tapping screw used for such a purpose has a threaded shaft provided with a drill bit portion having substantially the same outer diameter as that of the shank of the threaded shaft. By thrusting the tapping screw, the drill bit portion will penetrate through the outer wall and drill a starting hole in the light gage steel plate. The threaded shaft is then threaded into the starting hole while forming threads in the plate until the outer wall is clamped to the steel plate tightly by the head of the screw.
The resistance encountered when drilling a starting hole is proportional to the diameter of the drill bit portion. Thus, the larger the shaft diameter, the larger the thrust needed for making threads. In a conventional tapping screw, the shank of a threaded shaft and the drill bit portion have substantially the same outer diameters. Thus, the drill portion has a rather large diameter. If the tapping screw has a large diameter, e.g. 6 mm, then a large thrust has to be exerted on the screw for making threads. This worsens the working efficiency.
Further, forming threads with tapping screws is frequently done at a high place. Exerting a large thrust at a high place involves great danger. Also, it is difficult for a worker to exert such a large thrust over his head. Thus, the clamping operation can be difficult.